The Muscular System

Overview of Muscle Tissue

The muscular system is responsible for all types of body movement and is composed of three basic muscle types: skeletal muscle, cardiac muscle, and smooth muscle. Each type has distinct structural and functional characteristics.

• Skeletal muscle: Attached to bones, responsible for voluntary movements.

• Cardiac muscle: Found only in the heart, responsible for pumping blood.

• Smooth muscle: Located in walls of hollow organs, controls involuntary movements.

Key terminology: Prefixes myo- and mys- refer to "muscle"; sarco- refers to "flesh".

Comparison of Muscle Types

The three muscle types differ in structure, control, and location. The following table summarizes their main features:

Skeletal Muscle Structure

Connective Tissue Organization

Skeletal muscle fibers are organized and protected by layers of connective tissue:

• Endomysium: Encloses a single muscle fiber.

• Perimysium: Wraps around a fascicle (bundle) of muscle fibers.

• Epimysium: Covers the entire skeletal muscle.

• Fascia: Located outside the epimysium.

Muscles attach to bones via tendons (cordlike, mostly collagen) or aponeuroses (sheetlike structures).

Microscopic Anatomy of Skeletal Muscle

Skeletal muscle cells (fibers) have specialized structures for contraction:

• Sarcolemma: Specialized plasma membrane.

• Myofibrils: Long organelles inside muscle cell, composed of repeating units called sarcomeres.

• Sarcomere: The contractile unit, defined by Z discs; contains thick (myosin) and thin (actin) filaments.

• Sarcoplasmic reticulum (SR): Specialized smooth endoplasmic reticulum that stores and releases calcium ions.

Striations are due to alternating light (I bands) and dark (A bands) regions in myofibrils.

Organization of the Sarcomere

• Thick filaments: Composed of myosin proteins, with ATPase activity and myosin heads that form cross bridges during contraction.

• Thin filaments: Composed of actin proteins, anchored to Z discs.

• H zone: Central area of A band lacking actin filaments; disappears during contraction.

• M line: Center of H zone.

Muscle Contraction Physiology

Functional Properties of Muscle

• Irritability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to forcibly shorten when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil after stretching.

Nerve Stimulus and Action Potential

Skeletal muscles contract only when stimulated by a motor neuron. The motor unit consists of one motor neuron and all the muscle fibers it stimulates.

• Neuromuscular junction: Site where the axon terminal of the motor neuron meets the muscle fiber's sarcolemma.

• Neurotransmitter: Acetylcholine (ACh) is released into the synaptic cleft to initiate muscle contraction.

• Synaptic cleft: Gap between nerve and muscle, filled with interstitial fluid.

Steps of Muscle Activation

1. Nerve impulse reaches axon terminal.

2. Calcium channels open; Ca2+ enters axon terminal.

3. Ca2+ triggers release of ACh into synaptic cleft.

4. ACh binds to receptors on sarcolemma.

5. Sarcolemma becomes permeable to Na+; depolarization occurs.

6. Action potential is generated and spreads along muscle fiber.

7. ACh is broken down by acetylcholinesterase (AChE), ending contraction.

Restoration of resting state involves diffusion of K+ out of the cell and the action of the sodium-potassium pump.

Sliding Filament Theory

Muscle contraction occurs via the sliding filament mechanism:

• Ca2+ binds regulatory proteins on actin, exposing myosin-binding sites.

• Myosin heads attach to actin, forming cross bridges.

• Cross bridges pivot, pulling actin filaments toward the center of the sarcomere.

• ATP provides energy for repeated attachment and detachment of myosin heads.

Equation:

Types of Muscle Responses

Graded Responses

Muscle fibers contract in an "all-or-none" fashion, but whole muscles can produce graded responses by varying:

• Frequency of stimulation

• Number of muscle fibers stimulated

Types of responses include:

• Muscle twitch: Single, brief contraction.

• Summing of contractions: Rapid stimuli produce stronger contractions.

• Unfused (incomplete) tetanus: Partial relaxation between contractions.

• Fused (complete) tetanus: No relaxation; smooth, sustained contraction.

Muscle Force and Recruitment

Muscle tension increases as more fibers are stimulated. Maximum contraction occurs when all motor units are active.

Energy for Muscle Contraction

ATP and Regeneration Pathways

ATP is the direct energy source for muscle contraction, but is stored in limited amounts. Muscles regenerate ATP via three pathways:

Equation for aerobic respiration:

Muscle Fatigue and Oxygen Deficit

Prolonged or intense muscle activity can lead to fatigue, caused by:

• Ion imbalances (Ca2+, K+)

• Oxygen deficit and lactic acid accumulation

• Depletion of ATP

After exercise, rapid and deep breathing helps repay the oxygen deficit.

Types of Muscle Contractions

Isotonic vs. Isometric Contractions

• Isotonic contractions: Muscle shortens and movement occurs (e.g., bending the knee, lifting weights).

• Isometric contractions: Muscle tension increases, but muscle does not shorten (e.g., pushing palms together).

Muscle Tone

Muscle tone is a state of continuous partial contraction, resulting from alternating stimulation of motor units. It keeps muscles firm, healthy, and ready for action.

Effects of Exercise on Muscles

Exercise Adaptations

• Aerobic (endurance) exercise: Increases muscle strength, flexibility, and resistance to fatigue; improves metabolism, digestion, and coordination.

• Resistance (isometric) exercise: Increases muscle size and strength by enlarging individual muscle fibers (hypertrophy).

Example: Jogging and biking are aerobic exercises; weight lifting is resistance exercise.

Additional info: The notes are based on textbook slides and include inferred details for completeness and clarity.